This invention relates to arc welding power supplies, and, more specifically, to an improved apparatus providing a programmable arc welding power supply capable of operating in a constant current mode, constant voltage mode, constant power mode, or other welding modes without hardware reconfiguration, as well as providing great flexibility and capabilities not present in conventional systems.
In the past, it was not possible to use a single arc welding power supply to operate in a constant current mode and also a constant voltage mode without a hardware reconfiguration. A separate arc welding power supply would have to be used for each mode. Precise control of arc welding modes was not practical. The quality of a weld performed with an arc welding mode was dependent upon operator skill, and consistency of welds could be very difficult to obtain.
Some attempts have been made in the past to monitor a weld, sometimes in a crude fashion. For example, U.S. Pat. No. 4,093,844, issued to Fellure et al., represents an attempt to use an optical device to optically sense an arc, which is a crude way to monitor an arc welding procedure. Efforts to control arc length by optical scanning have been less than satisfactory. It is hardly possible to maintain a constant voltage during arc welding, or to maintain a constant current during arc welding, using nothing more than optical scanning to monitor the weld.
In the past, attempts have been made to tap the primary of the transformer used in a welding power supply. For example, U.S. Pat. No. 4,024,371, issued to Drake, represents an attempt to simply monitor a weld, and to measure the power factor on the primary of the transformer. The impedance of the weld is monitored. A computer is proposed to monitor the weld, compute statistical information concerning the weld, compute statistical information concerning the weld, and if the impedance value measured for the weld is outside certain user defined limits, the computer may indicate to an operator that the weld should be rejected. The Drake device is limited to use in connection with a pulse type welder. Drake proposes the use of a timing circuit and a clock circuit which generates pulses which are counted as a means for timing a period which is assumed to be sufficient for a particular welding procedure. The technique of counting pulses in a pulse welder or resistance welder is an unsatisfactory means for controlling power output in an arc welding device.
Others have attempted to use a microprocessor's cycle time or time for executing one instruction cycle for timing purposes, the microprocessor becoming little more than a digital timer. Such attempts have often involved the use of the microprocessor to count weld cycles. An example of such a device is proposed in U.S. Pat. No. 4,104,724, issued to Dix et al. It is unsatisfactory to have to program a welding procedure in terms of number of weld cycles, because the number of weld cycles necessary may vary for a given weld, must be determined empirically, and such control techniques are totally generally inapplicable to arc welding modes. The Dix device is a spot welder, using a single phase alternating current power supply. Such control techniques are not applicable to three-phase direct current arc welding power supplies. In Dix, significantly, the illustrative microprocessor is not inside a feedback loop, but is generally used as a sophisticated timer.
There is a need for a programmable arc welding power supply, capable of controlling three phase direct current arc welding power supplies. There is a need for a programmable controller which senses current, voltage, or both, directly (instead of attempting to measure the impedance of a weld), and which is capable of adjusting the circuit parameters in a manner which causes the current, or voltage, or both, to conform to program control. There is a need for a programmable control system which directly measures current, voltage, or both and computes the first derivative of the welding current or voltage function and uses that computation as an indication of the rate of change of the welding current or voltage, which may then be used for positive control of the current or voltage, or both.
In the past, it has been necessary to completely rewire a device if it was necessary to switch from, for example, a constant voltage mode to a constant current mode. There is a need for a programmable arc welding power supply that is capable of operating in a constant voltage mode, a constant current mode, or even a constant power mode, without requiring the circuit to be rewired. There is a need for a single welding power supply which is capable of operating in all common arc welding modes.
In the past, robot devices, which are typically digital, have had to interface with welding devices, which were analog or only accepted analog input. Control has often been accomplished by translating information to a zero to ten volt analog signal, for example, which is then sent from one device to the other where the analog signal must be used, or perhaps translated back to a digital signal. This has not provided precise positive control or communication between the robot device and the welding device. Such an approach requires precise calibration of the analog to digital conversion process, and may be susceptible to the introduction of noise and errors. There has been a need for a programmable arc welding power supply controller which is capable of accomplishing direct communication to a robot device using digital data.
Conventional methods of starting a TIG weld have involved the use of high frequency pulses at voltages on the order of 15,000 to 20,000 volts. Such methods tend to cause electromagnetic interference (EMI) or radio frequency interference (RFI). EMI or RFI can interfere with the proper operation of robots, as well as other devices and instruments. There is then a need for a TIG start weld method which avoids the use of high frequency, high voltage pulses.
Other conventional attempts to start TIG welding procedures have involved the use of what is commonly referred to as a scratch start. The tungsten tip of a welding lead is quickly scratched across the metal to be welded in order to start an arc. This must be done quickly in order to avoid the tungsten tip from being welded to the metal work piece. This procedure often damages the tungsten tip and tends to contaminate the metal work piece with tungsten.
There is a need in the art for an intelligent or smart arc welding power supply which is capable of sensing contact with the metal work piece, capable of sensing the establishment of an arc utilizing a background supply, and capable of ramping up the main power supply current upon establishment of an arc.